There are various sound waves with different frequencies. Noises from the factories, trestles or highways annoy people the most. There are many ways of noise reduction by using, for example, porous glass wool or dual-layered structures. Recently, a phononic crystal is used to reduce noise due to the band gap of a phononic crystal, which can be used in a bulk acoustic wave filter or a surface sonic wave filter to prevent a incoming sonic wave with a specific incoming angle and frequency to achieve noise reduction.
However, noise reduction is relatively passive. In this disclosure, in addition to noise reduction, the acoustic energy of a sound wave can be converted into valuable energy in other format to further achieve environmental protection.